The invention generally relates to automated anti-skid braking systems. More specifically, the invention concerns control methods for determining which of a number of available brake commands will be issued at a given point in time during an anti-skid braking sequence.
Automatically controlled anti-skid braking systems (ABS) traditionally have sought to attain three goals. The first goal of an ABS is to avoid front wheel lock-up. Under such a condition, one loses "steerability." The second goal of a typical ABS is to avoid "fish-tailing" or rear-end stability. The third goal of an ABS is to minimize the stopping distance. It has been found that the stopping distance of a vehicle may be made shorter if the wheels are operated at low slip rather than in a fully locked or skid condition (the effective coefficient of friction is greater at lower slip than at full slip).
The typical ABS attempts to optimize stopping distance, steerability and rear-end stability during so-called "panic stops". In a typical ABS method, one desires a high brake torque "apply" rate for quick response. Additionally, one needs a high "release" rate, if the condition of lock-up is sensed as about to begin. The conditions of "apply", "hold" and "release" refer respectively to increasing, constant and decreasing brake pressure or resulting brake torque. The apply state means brake torque is being increased, the release state means that brake torque is being decreased, while the hold state indicates that the brake torque is being maintained constant.
In most control systems there is a desire for large rates of change when the controlled state is far from its desired value and for small rates of change when the controlled state is close to its desired value. Also, it is undesirable to have large swings between the apply and release states, due to limitation of typical hydraulic braking systems. Hence, any chosen ABS control law should not go back and forth at high rates between the apply and release states, else a requirement for larger hydraulic components will arise.
One known solution for producing smaller rates of change which avoid such large swings between apply and release is to use the so-called "step-up" and "step-down" approach. The step-up and step-down approach basically interposes a hold state between any apply and release sequence. For example, under a step-up, one would enter an apply state, then enter a hold state perhaps longer than the apply state before entering another apply state, and alternating thereafter. Functionally, such an approach mimics a slow apply condition. Conversely, in a step-down, one would have a release state followed by a hold state and so on to provide a type of "slow release".
In the prior art, most logic required to implement the control law for an ABS is used in formulating step-up and step-down sequences, which may involve many special cases which need special timers and control paths along with storage or saving of previous control states and a history of their occurrence over time. Additionally in the prior art, the phase plane of wheel slip versus angular acceleration of the wheel utilized a great number of control areas in the plane, each of which called for one of several different actions on the part of the braking system.
Therefore, there is a need for an anti-skid braking method using a control law which will result in a savings of the logic required for its implementation.